Precision VBS (variable bleed solenoid) devices are commonly used to provide accurate hydraulic control of certain automatic transmission functions. For example, one industry standard function has involved regulating the main transmission supply pressure.
Fluid pressure to control respective clutches of an automatic transmission can be controlled using VBS devices. For example, several VBS devices have been mounted/packaged on a common control module assembly to provide advanced transmission clutch control logic as described, for example, in U.S. Pat. No. 6,695,748. This technology has required additional precision and repeatability of the VBS pressure control characteristics.
Traditionally, the VBS pressure control algorithm uses transmission oil temperature as a primary input to the control strategy (either a series of lookup tables or as a system model) to derive the output pressure control. In the past, a single common transmission oil temperature sensor (e.g. thermistor) has been used as the key input. The drawbacks to this approach are that the single thermistor is usually packaged away from at least some of the VBS devices of the transmission pressure control and that there is generally a significant temperature difference in the actual VBS internal temperature and the thermistor temperature. As a result, the system model accuracy is compromised. The condition is amplified if there are multiple VBS solenoids and if the arrangement of solenoids is exposed to varying transmission sump oil levels. These conditions cause varying thermal losses (convection) and subsequent changing and/or varying VBS temperatures. The internal solenoid temperatures vary due to oil levels, solenoid duty cycle and the like and render precise modeling difficult to attain. That is, the average remote measurement of the single thermistor yields a temperature difference from the actual solenoid temperatures involved, and the temperature difference varies by solenoid positions and/or oil level such that resultant modeled pressure control inaccuracies are experienced.